Plasma-enhanced chemical vapor deposition (PECVD) can be used to lay down high quality silicon on a variety of substrates. Lower workpiece temperatures can be used when the usual chemical vapor deposition (CVD) is augmented with a hydrogen plasma. Such plasma is typically heated between radio frequency (RF) electrodes, sometimes supplemented with a strong magnetic field to contain the plasma. The RF power, excitation frequency, silane (SiH.sub.4) feed fraction, total gas pressure, gas composition (H.sub.2 only or H.sub.2+Ar) and electrode configuration all have some influence on the film crystallinity and growth rate.
In a prior art PECVD system, silane gas is introduced perpendicularly through showerheads to the plasma and it initially concentrates to about 7% silane in hydrogen. The silane concentration (SC) drops to about 5% as the silicon is deposited from the silane and is laid down on the workpiece. The depleted process gases are removed. The conversion of silane to silicon (SiH.sub.4.fwdarw.Si+2H.sub.2) frees additional hydrogen that must be pumped off to maintain the optimal pressure in the processing chamber. Unfortunately, the 5% silane is removed as well because such a low SC will adversely affect the quality of the silicon thin-film deposition. A lot of silane is wasted in conventional PECVD processing equipment in attempting to keep the wash of process gases at 6-7% silane concentration.
The bright promise of alternative forms of energy such as solar electric has not been fully accessed because the costs of solar photovoltaic cells and devices have been so high. A 100-watt solar electric panel can easily cost $500 or more. Gasoline, propane, and diesel driven electric generators still continue to be generally more attractive in terms of capital costs. Staying connected to the electric utility grid is still the only viable energy alternative for the vast majority of homes and businesses.
The cost of photovoltaic devices is too high because of the manufacturing methods currently in use. As a result, photovoltaic devices are finding only niche applications. Some of the high costs of photovoltaic devices have been the result of the tremendous waste of silane in conventional PECVD processes.
There are several conventional ways that photovoltaic devices are being presently fabricated. For example, new single crystal silicon wafers, scrap or re-claimed single crystal silicon, and thin-film deposition of photovoltaic devices on inexpensive substrates.
A typical thin-film photovoltaic device comprises several layers including a substrate, a barrier layer to isolate the thin-films from the substrate, an indium tin oxide or tin oxide transparent-conductor, a PIN photodiode structure, a second conductor such as ZnO and a metal conductor and reflector to trap light, an environmental protective coating, and a mounting system for strength and easy installation.
Plasma-enhanced chemical vapor deposition (PECVD) in a mostly hydrogen atmosphere is a conventional method for fabricating silicon, thin-film, PIN photodiode structures. Hydrogen atoms and radicals produced by the hydrogen plasma incorporate into the deposited silicon for good hydrogen passivation of the grain boundary defects and other dangling bonds. Such passivation is critical for the proper functioning of amorphous silicon and nano-crystalline PIN-diode structures.
The PECVD process generates a hydrogen plasma above a substrate using strong radio-frequency (RF) fields. The substrate is often heated, e.g., 200.degree. C. A dilute silane (SiH), e.g., in concentrations of 5-10%, is introduced into the plasma and the result is a deposit of thin-film silicon on the substrate. Different RF frequencies are used, e.g., the industry standard 13.56 MHz, 95 MHz, and various microwave frequencies, each with their own advantages and disadvantages.
The typical PIN-diode structures fabricated with PECVD have typical sunlight-to-electricity conversion efficiencies of about 7% after several months of stabilization in sunlight. Unfortunately, the conventional PECVD deposition rates are slow, e.g., about 0.5 nm per second. Such slow rates are responsible for much of the high production costs of PIN-diode photovoltaic devices, as expressed in terms of fabrication costs per square meter, or per the peak power that can be generated by the device.
The efficiency of conversion of silane-to-deposited-silicon in conventional PECVD processes is poor, typically in the range of 5-15%. Most of the silane is wasted in the exhaust because it is simply blown through in an attempt to maintain the uniformity and the quality of the silicon being deposited. Prior art attempts to increase the deposition rates of silicon from silane using PECVD and hot-wire techniques have increased the deposition rates, but the quality of the deposited silicon was too poor to get high electric-conversion efficiency thin-film PIN-diode photovoltaic devices.
In a standard PECVD deposition system, the gas mixture (e.g. 6.5% silane in hydrogen); is introduced at one part of the chamber and then removed at another. A large flow rate is passed through the deposition chamber so that the silane concentration is not excessively depleted and reasonably uniform deposition parameters can be achieved (FIG. 2). For example, assume that the desired concentration of silane is 6.5%. To obtain the minimum possible flow rate of the gas mixture and the maximum utilization of silane, one would input a gas mixture that had 13% silane. As the gas mixture flowed through the plasma, the silane is consumed by the deposition such that the concentration at the output gas mixture is 0%. (See FIG. 2a) The total amount of input gas is just enough to supply the amount of silane consumed. Unfortunately, most deposition processes will not produce adequate silicon quality on the substrate with such a large variation in silane concentrations.
In standard PECVD systems, a much larger flow rate is used, instead. For example, the input mixture might be 7% silane and the flow rate set so that the concentration at the output is depleted to 6% silane. Such a high flow rate might provide adequate uniformity of the deposition parameters, but as a result, 6/7 of the silane is pumped out and wasted (FIG. 2). Tighter control of the variation in silane concentration would require yet higher flow rates and more waste. (For example, input 7%, output 6.5% would require twice the flow rate as above and would waste 6.5/7 or 93% of the silane). Thus, the material utilization is typically in the range of (5-15%).
In the formation of silicon films by CVD methods, there are additional problems. For example, production yield is low due to contamination of apparatuses or generation of foreign materials caused by silicon particles formed in a gas phase since gas phase reactions occur in CVD processes. A film having uniform thickness is difficult to obtain on a surface having concavo-convex areas since starting materials are gases. Productivity is low since the growth rates of films are low, and complicated and expensive high-frequency generators and vacuum apparatuses are required for plasma CVD. Accordingly, further improvements of formation of silicon films have been strongly desired.
In addition, handling of starting materials is difficult since not only are the gaseous forms of silicon hydrides used for CVD toxic and strongly reactive, but also a require a sealed vacuum apparatus. In general, the apparatus mentioned above is not only large and expensive, but also the vacuum system and/or plasma generation system of apparatus consumes large amounts of energy, resulting in further increases production cost.